Thermal conductivities of irradiated UO2 and (U, Gd)O2 pellets

Thermal diffusivities of UO2 and (U, Gd)O2 pellets irradiated in a commercial reactor (maximum burnups: 60 GWd/t for UO2 and 50 GWd/t for (U, Gd)O2) were measured up to about 2000 K by using a laser flash method. The thermal diffusivities of irradiated UO2 and (U, Gd)O2 pellets showed hysteresis phenomena: the thermal diffusivities of irradiated pellets began to recover above 750 K and almost completely recovered after annealing above 1400 K. The thermal diffusivities after recovery were close to those of simulated soluble fission products (FPs)-doped UO2 and (U, Gd)O2 pellets, which corresponded with the recovery behaviors of irradiation defects for UO2 and (U, Gd)O2 pellets. The thermal conductivities for irradiated UO2 and (U, Gd)O2 pellets were evaluated from measured thermal diffusivities, specific heat capacities of unirradiated UO2 pellets and measured sample densities. The difference in relative thermal conductivities between irradiated UO2 and (U, Gd)O2 pellets tended to become insignificant with increasing burnups of samples.